Such detectors area already known within various fields of activity, from sensing objects passing by on a continuous conveyer to sensing particles in flowing air or liquid media, see e.g. European patent applications Nos. 0 021 163 and 0 194 243. A problem said to be solved by these known detectors is that their operational signal will be compensated for changes dependent on ageing of the components used, temperature variations and dirtyness of the radiation source and/or the detector element.
In connection with a digitilized smoke detector it has been proposed that a separate signal is taken out for indicating that the function degree of the detector is restricted to an inacceptable level, see e.g. U.S. Pat. No. 4,317,113. The known detector having dirtyness compensation and level detection includes a rather complicated circuit arrangement. Furthermore, the signal indicating the dirty degree is taken out at a point downside of a special comparator unit to be able to be used as a function alarm via a complicated circuit net. It involves a complicated and uneconomic arrangement for obtaining an accurate level indication.